NOx absorber catalyst (NAC) are known e.g. from U.S. Pat. No. 5,473,887 (the entire contents of which is incorporated herein by reference) and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. diesel fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, the oxygen concentration is adjusted to a desired redox composition intermittently in response to a calculated remaining NOx absorption capacity of the NAC, e.g. richer than normal engine running operation (but still lean of stoichiometric or lambda=1 composition), stoichiometric or rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold. More sophisticated common rail fuel injector systems in diesel engines can be used to meter very precise quantities of fuel to adjust exhaust gas composition.
A typical NAC formulation includes a catalytic oxidation component, such as platinum, a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (1);andBaO+NO2+½O2→Ba(NO3)2  (2),wherein in reaction (1), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+3/2O2 or Ba(NO3)2→BaO+2NO2+1/2O2  (3);andNO+CO→½N2+CO2 (and other reactions)  (4).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide. Equally, the skilled person can adapt the reaction scheme for NOx absorber components other than barium, eg. other alkaline earth metals or alkali metals.
Increasing concern about the environment, and increasing fuel prices, has led to the introduction of ever larger numbers of diesel engines for motor cars and light commercial vehicles. The emission control regulations now include strict control of “soot” or particulate matter (“PM”), as well as CO, hydrocarbons (“HC”) and NOx. For control of PM, it has become clear that a filter or trap is required to remove PM from the flowing exhaust gases. One form of filter is commonly known as a wall-flow filter, whose construction is well known to the skilled person.
Practical wall flow filters are generally catalysed, usually with a catalyst to reduce PM combustion temperature and/or an oxidation catalyst capable of catalysing the conversion of NO in the exhaust gas to NO2, for the NO2/PM reaction.
WO 01/12320 discloses a wall-flow filter for an exhaust system of a combustion engine comprising: an oxidation catalyst containing e.g. a platinum group metal on a substantially gas impermeable zone at an upstream end of open upstream channels; and a gas permeable filter zone downstream of the oxidation catalyst for trapping soot. The downstream channels of the filter can include a NOx absorber catalyst (NAC) and optionally a Selective Catalytic Reduction (SCR) catalyst downstream of the NAC.
WO 2004/022935 discloses an exhaust system for a lean-burn internal combustion engine comprises a nitrogen oxide (NOx) absorbent, a catalyst for catalysing the selective catalytic reduction (SCR) of NOx with a NOx specific reactant e.g. ammonia, first means for introducing a NOx specific reactant or a precursor thereof, e.g. urea, into an exhaust gas upstream of the SCR catalyst and means for controlling the introduction of the NOx-specific reactant or the precursor thereof into the exhaust gas via the first introducing means, wherein the SCR catalyst is disposed upstream of the NOx absorbent and optionally with the NOx absorbent, and wherein the control means is arranged to introduce the NOx-specific reactant or the precursor thereof to exhaust gas via the first introducing means only when the SCR catalyst is active, whereby exhaustion of NOx-specific reactant to atmosphere is substantially prevented.
U.S. Pat. No. 7,062,904 discloses a filter coated with a NOx adsorber/catalyst on inlet sides of filter elements and SCR catalyst on the outlet sides of the filter elements. The adsorber/catalyst preferably enriches the ratio of NO2 to NO in the NOx it does not adsorb. It is clear from the description that the catalyst can be combined with the NOx adsorber or it can be separate from and upstream of the NOx adsorber: it cannot be both combined with the NOx adsorber and located upstream of the NOx adsorber.
DE 10 2005 005 663A1 discloses that a wall-flow filter may carry a NOx trap, or NOx absorption catalyst (NAC), coating on the inlet cells, and an SCR catalyst coating on the exit cells. It is not clear that this design has ever been commercialised.
A problem with the wall-flow filter disclosed in DE 10 2005 005 663 A1 is that it can lead to increased emissions of ammonia following NAC regeneration events and is poor at treating cold-start emissions e.g. from cold-start from the first ECE cycle of the New European Drive Cycle (NEDC).